Formation And Properties Of Ti-Based Bulk Metallic Glass

Ti-bassed bulk metallic glass(BMG) presents excellent application perspective due to its high mechanical strength and high glass forming ability(GFA). In this thesis, the Ti-based alloy is studied, the Ti₄₀Zr₂₅Cu₉Ni₈Be₁₈ BMG is designed, the GFA, the thermal stability, the crystallization kinetics, the mechanical properties as well as the effect of element doping on its properties are studied systematically. In the same time, a new method, the phase transition cooling (PTC) technology is used to prepare BMG, the thermal and mechanical properties are studied as well.By copper mold casting method, the Ti₄₀Zr₂₅Cu₉Ni₈Be₁₈ BMG with good GFA is prepared. The thermal stability and crystallization kinetics of the alloy are studied by differential scanning calorimetry (DSC) at various heating rates, the glass transiton temperature (Tg), the crystallization temperature(Tx) of various peaks as well as the crystallization activation energy were obtained. The crystallization procedure of the BMG was observed by in situ X-ray study, the mechanical property of the BMG is studied by single-axis compressive test at room temperature.The effect of Yittrium doping on the properties of the copper mold casting BMG is studied. With 1 at.% addition of Y, the alloy rod with 3 mm in diameter presents amorphous nature by X-ray examination, with 2 at.% addition of Y, minor crystalline peaks can be observed in the XRD curve of the alloy, with the increase of Y addition, more crystalline peaks can be observed in the XRD pattern, which shows that the doping of Y can not improve the GFA of the alloy. The mechanical properties of the 1, 2 and 5 at.% Y doped alloy sample were studied by single-axis compressive test at room temperature, also the fracture morphology of the samples were observed by Scanning electron microscropy (SEM). The results show that, with 1% of Y doping, the full glass sample presents fracture feature with the plastic strain being 1%; with 2% of Y doping, the sample with partial crystalline phase presents no plastic strain, the sample presents quasi-cleavage feature; with 5% of Y doping, the crystalline sample presents no plastic strain and cleavage feature. (Ti₄₀Zr₂₅Cu₉Ni₈Be₁₈)_100-xNb_x(x=0, 1, 2, 3, 4, 5 at.%) BMG rods with 3 mm in diameter were prepared by copper mould casting. The thermal stability and the crystallization kinetic of the BMG samples were studied by DSC at various heating rates. The thermal stabilility of the alloy decreases with the increase of Nb.Single-axis compressive test for the Nb doped alloy shows that, with 3 at% of Nb addition, the fracture strength is 2177 Mpa, and the plastic strain is 13 %, which reveals superior mechanical property; with 4 at% of Nb addition, the fracture strength is 1934 Mpa, the plastic strain is 8%. Further studies were carried out to understand the improve of Nb addition on the mechanical property of the alloy: during the compressive deformation, the atoms of Nb transport and enrich in the shear bands, which prevent the propagation of the shear band and favor the nucleation of shear bands,branching of the shear bands, and change the develop direction of the shear bands, and further improve the plastic of the alloy.The max cooling rate of the PTC technology is 300 k/s. A Ti₄₀Zr₂₅Cu₉Ni₈Be₁₈ BMG rod with the diameter is prepared successfully by PTC technology. The samples show amorphous nature examined by XRD and DSC test. Compressive mechanical test of the samples show that the fracture strength is as high as 2636 Mpa, the plastic strain being 37%; those of the samples with the same composition and the same sample dimension by copper mold casting is 1643 Mpa and 8%, respectively. Further studies show that with higher cooling rates, more free volume was obtained in the samples prepared by PTC technology than in those prepared by copper mold casting, which favors the formation of the shear bands, and improve the plasticity of the alloy finally.